In a large microwave-safe bowl, add onion slices and mushrooms. Cook in microwave for about 4 minutes, until very soft.
To make the patties: In a large bowl, combine ground beef, chopped onions, bread crumbs, egg whites, horseradish and pepper. Shape into 5 patties. Using a large nonstick pan coated with cooking spray, cook patties over medium-high heat for about 6 minutes, spray the top of patties with cooking spray, turn over, turn heat down to medium and cook an additional 5-6 minutes until no longer pink inside. Remove patties to a plate.
To make the gravy: In the same pan, (not cleaned) melt butter. Using a rubber spatula, stir in flour, scraping bottom of pan. Add salt, pepper and cook for 2 minutes while stirring often. Stir in milk to form a thick paste. Continue to stir for about a minute. Slowly stir in chicken broth, turn heat to medium-high and cook until thicken. Continue to stir the entire time. Add Worcestershire sauce and several drops of Tabasco sauce and mix well. Turn heat down to low and simmer for 5 minutes. Stir often.
To the gravy, add mushrooms/onions and the liquid from the bowl and the patties. Heat for about 5 minutes until heated through.
To serve: Spread out ½ cup mashed potatoes onto each plate. Place 1 meat patty on top of potatoes. Top with about ⅔ cup gravy. If serving with baked potatoes, once baked, open the potato and mash with a fork. Top the potato with 1 patty and gravy. It’s also delicious served on top of a slice of bread or just on a plate with no potatoes or bread.
Any leftovers freeze great. Pack 1 patty and ⅔ cup gravy into individual plastic contains for the freezer.